Conventionally, gradient index rod lenses are used in collimators for communication applications, optical coupling elements used in optical fibers, endoscope objective lenses for medical applications, objective lenses used in optical disk systems such as CD players and DVD players.
Such a gradient index rod lens is made of a cylindrical transparent element which has a refractive index continuously varying from the center to the periphery thereof and which is known as a converging light-transmitting body for which the refractive index n(r) at a position a distance r from the central axis in the radial direction is given approximately by the quadratic equation in r,n(r)=n0{1−(g2/2)·r2},wherein n0 represents the refractive index at the central axis, and g represents the square distribution constant.
If the length z0 of the rod lens is chosen to be in a range of 0<z0<π/2g, then the image formation characteristics of the rod lens will be the same as those of a normal convex lens, even though the both end faces of the rod lens are flat; when a parallel light beam is incident on one end face of the rod lens, a focal point will be formed at a position a distance so from the other end face of the rod lens (the end face from which the light beam exits), wheres0=cot(gz0)/n0g.
The gradient index rod lens having such characteristics can be used in the form of a cylinder, and therefore can be easily incorporated in various kinds of apparatuses. Further, the both end faces of the gradient index rod lens are flat surfaces orthogonal to the optical axis of the rod lens, and optical axis alignment can be easily carried out for optical systems.
Such a gradient index rod lens can be manufactured by the following method, for example:
A rod-shaped element is formed from a glass having 57 to 63 mol % of SiO2, 17 to 23 mol % of B2O3, 5 to 17 mol % of Na2O, and 3 to 15 mol % of Tl2O as principal components. This glass rod element is then treated in an ion exchange medium such as a potassium nitrate salt bath, thus carrying out ion exchange between thallium ions and sodium ions in the glass and potassium ions in the medium, and hence giving the glass rod element a refractive index distribution in which the refractive index decreases continuously from the center of the glass rod element toward the periphery thereof. According to this manufacturing method, even a gradient index rod lens with a diameter of not more than 1 mm can be easily manufactured at low costs. Further, a gradient index rod lens having the same characteristics may be manufactured even from a transparent plastic instead of glass.
Suitable applications of the gradient index rod lens include microchemical systems as one of integration technologies for carrying out chemical reactions. Such a microchemical system is intended to have capability of carrying out all functions of mixing, reaction, separation, extraction, detection or the like on a sample placed in a very narrow channel which is formed in a small glass substrate or the like. A microchemical system having a single function such as separation, or a microchemical system having a plurality of functions may be used. Examples of reactions carried out in the microchemical system include diazotization reactions, nitration reactions, and antigen-antibody reactions. Examples of extraction/separation include solvent extraction, electrophoretic separation, and column separation.
As an example in which ‘separation’ is the sole aim, an electrophoresis apparatus for analyzing extremely small amounts of proteins, nucleic acids or the like has been proposed by Japanese Laid-open Patent Publication (Kokai) No. 8-178897. This electrophoresis apparatus analyzes extremely small amounts of proteins, nucleic acids or the like and is provided with a channel-formed plate-shaped element comprised of two glass substrates joined together. Because the element is plate-shaped, breakage is less likely to occur than in the case of a glass capillary tube having a circular or rectangular cross section, and hence handling is easier.
In the microchemical system, because the amount of the sample is very small, a high-precision detection method is essential. As such a high-precision detection method, a photothermal conversion spectroscopic analysis method has been established, which utilizes a thermal lens effect that is produced through a liquid-borne sample absorbing light in a very narrow channel. The path to making a detection method of the required precision fit for practical use has been opened up through the establishment of the above analysis method.
The photothermal conversion spectroscopic analysis method utilizes a photothermal conversion effect that when light is convergently irradiated onto a sample, the temperature of a solvent is locally increased by thermal energy emitted due to light absorbed by a solute in the sample to cause a change in the refractive index and hence generate a thermal lens.
FIG. 5 is a view useful in explaining the principle of a thermal lens.
In FIG. 5, a convergent beam of exciting light is irradiated onto an extremely small sample via an objective lens of a microscope, whereupon a photothermal conversion effect takes place. For most substances, the refractive index drops as the temperature rises, and hence the drop rate of the refractive index of the sample is greater toward the center of the convergent beam of exciting light, which is where the temperature rise is highest. Due to thermal diffusion, the temperature rise becomes smaller and hence the drop in refractive index becomes smaller, with increasing distance from the center of the convergent beam of exciting light, i.e. decreasing distance to the periphery of the same. Optically, this pattern of change in the refractive index brings about the same effect as with a concave lens, and hence the effect is called the thermal lens effect. The size of the thermal lens effect, i.e. the power of the thermal lens is proportional to the optical absorbance of the sample. Moreover, in the case that the refractive index increases with temperature, a converse effect to the above, i.e. the same effect as a convex lens is produced.
In most cases where the photothermal conversion spectroscopic analysis method using the thermal lens described above is carried out, it is required that the focal position of the exciting light and that of the detecting light should be different from each other. FIGS. 6A and 6B are views useful in explaining the formation position of the thermal lens and the focal position of the detecting light in the direction of travel of the exciting light. FIG. 6A shows a case in which the objective lens has chromatic aberration, whereas FIG. 6B shows a case in which the objective lens does not have chromatic aberration.
In measurement according to the photothermal conversion spectroscopic analysis method using the thermal lens, in the case that the objective lens 130 has chromatic aberration, a thermal lens 131 is formed at the focal position 132 of the exciting light as shown in FIG. 6A. The focal position 133 of the detecting light is shifted by an amount ΔL from the focal position 132 of the exciting light, and thus changes in the refractive index within the thermal lens 131 can be detected as changes in the focal distance of the detecting light from the detecting light. In the case that the objective lens 130 does not have chromatic aberration, on the other hand, the focal position 133 of the detecting light is almost exactly the same as the focal position 132 of the exciting light, i.e. the position of the thermal lens 131 as shown in FIG. 6B. The detecting light is thus not deflected by the thermal lens 131, and hence changes in the refractive index within the thermal lens 131 cannot be detected.
There is the optimal value for the difference between the focal position of the exciting light and the focal position of the detecting light. This optimal value is determined by the wavelengths of the exciting light and the detecting light, the intensity of the exciting light and the detecting light, the concentration of the sample, the thickness of the sample, etc. The different ΔL between the focal position of the exciting light and the focal position of the detecting light is desirably Ic<ΔL<30·Ic.
The confocal length Ic (nm) is given by Ic=π·(d/2)2/λ1, wherein d represents the diameter of the Airy disk and is given by d=1.22×λ1/NA, λ1 represents the wavelength (nm) of the exciting light, and NA represents the numerical aperture of the lens.
The optimal value of the difference ΔL described above varies according to the thickness of the sample to be analyzed. When carrying out measurements on a sample having a thickness lower than the confocal length, it is more preferable for ΔL to be equal to Ic<ΔL<20·Ic, and it is most preferable for ΔL to be equal to √3·Ic. Therefore, it is desirable that the objective lens should have such a chromatic aberration that the ΔL value is close to the optimal value.
However, there is a limitation on the type of ions (glass components) that can be used to prepare a gradient index rod lens as described above. For example, thallium, lithium, cesium, and silver are frequently used so that a desired value of chromatic aberration cannot be always obtained. The chromatic aberration of the gradient index rod lens largely depends upon the type of ions used, though it also depends upon the type of mother glass used. A lens having a predetermined range of chromatic aberration can be manufactured using each type of ion, but there can be a range of chromatic aberration between predetermined ranges of chromatic aberration, that cannot be obtained.
Therefore, in the case where measurements are carried out according to the photothermal conversion spectroscopic analysis method in a microchemical system as described above, there is a fear that the chromatic aberration of the gradient index rod lens can assume an unsuitable value depending upon the conditions, and hence measurements cannot be properly carried out.
It is a first object of the present invention to provide a gradient index rod lens unit having a desired chromatic aberration, and further provide a gradient index rod lens unit which can permit varying only the aperture number of the focal position without varying the chromatic aberration and the distance between an end face of the lens and the focal position thereof.
It is a second object of the present invention to provide a microchemical system which is provided with a gradient index rod lens unit having a desired chromatic aberration.